So-called "M & E cells" are electrochemical cells employing a membrane and electrode structure. Such cells can be operated as an electrolytic cell for the production of electrochemical products, or they may be operated as fuel cells for the production of electrical energy, gas generating devices and processes, chemical synthesis devices, chemical treatment and processing devices and methods, gas dosimeters and sensing devices and the like. Electrolytic cells may, for example, be used for the electrolysis of an alkali metal halide such as sodium chloride or for the electrolysis of water. M & E cells are well known in the art.
The contact of the gas-liquid permeable porous electrode with the ion exchange membrane is an important factor for the efficiency of the M & E cell. When the thickness of an electrode is nonuniform or the contact between the electrode with the ion exchange membrane is not satisfactory, a part of the electrode is easily peeled off adversely effecting the electrical properties of the cell. The advantages of the M & E structure are then decreased or lost.
Membrane and electrode structures are currently manufactured by several techniques. U.S. Pat. No. 3,297,484 illustrates in detail materials for electrode structures including exemplary catalyst materials for electrodes, ion exchange resins for membrane and electrode structures and current collecting terminals. Catalytically active electrodes are prepared from finely-divided metal powders, customarily mixed with a binder such as polytetrafluoroethylene resin. The electrode is formed from a mixture of resin and metal bonded upon one or both of the surfaces of a solid polymer matrix, sheet or membrane.
In U.S. Pat. No. 3,297,484, the mixture of resin and catalytically active particles is formed into an electrode structure by forming a film from an emulsion of the material, or alternatively, the mixture of resin binder and catalytically active particles is mixed dry and shaped, pressed and sintered into a sheet which can be shaped or cut to be used as the electrode. The mixture of resin and catalytically active particles may also be calendered, pressed, cast or otherwise formed into a sheet, or fibrous cloth or mat may be impregnated and surface coated with the mixture. In U.S. Pat. No. 3,297,484, the described electrodes are used in fuel cells. In U.S. Pat. No. 4,039,409, the bonded electrode structure made from a blend of catalyst and binder is used as the electrode in a gas generation apparatus and process.
In U.S. Pat. No. 3,134,697, many ways are described for incorporating catalytically active electrodes into the surfaces of an ion exchange membrane. In one embodiment, as explained above, the electrode material made of catalytically active particles and a resin binder may be spread on the surface of an ion exchange membrane or on the press platens used to press the electrode material into the surface of the ion exchange membrane, and the assembly of the ion exchange membrane and the electrode or electrode materials is placed between the platens and subjected to sufficient pressure, preferably at an elevated temperature, sufficient to cause the resin in either the membrane or in admixture with the electrode material either to complete the polymerization if the resin is only partially polymerized, or to flow if the resin contains a thermoplastic binder.
It is known to add binders, such as fluorocarbon polymers including polytetrafluoroethylene and polyhexylfluoroethylene, to the electrode ink. It is also known to add viscosity regulating agents such as soluble viscous materials to the electrode ink.
A method to construct membrane and electrode structures is also described in "Methods to Advance Technology of Proton Exchange Membrane Fuel Cells;" E. A. Ticianelli, C. Derouin, A. Redondo and S. Srinivasan presented at Second Symposium "Electrode Materials and Processes for Energy Conversion and Storage," 171st Electrochemical Society Meeting, May, 1987. In this approach, a dispersion of a flocculent precipitate of 20% platinum on a catalyst and TEFLON.RTM. (commercially available from E. I. du Pont de Nemours and Company) is prepared. The flocced mixture is cast onto paper and then pressed onto a carbon paper substrate. The electrodes may then be sintered at elevated temperature, approximately 185.degree. C., for 30 minutes. The electrode is next brushed with a solution of chloroplatinic acid and subsequently reduced with an aqueous mixture of sodium borohydride.
The electrode is then washed and NAFION.RTM. (commercially available from E. I. du Pont de Nemours and Company) solution brushed on the surface of the electrode. The method of solution processing is described in "Procedure for Preparing Solution Cast Perfluorosulfonate Ionomer Films and Membranes," R. B. Moore and C. R. Martin, Anal. Chem., 58, 2569 (1986), and in "Ion Exchange Selectivity of NAFION.RTM. Films on Electrode Surfaces," M. N. Szentirmay and C. R. Martin, Anal. Chem., 56, 1898 (1984). The so-called NAFION.RTM. solution may be made from a solvent which is, for example, a lower-boiling alcohol such as propanol or a high-boiling alcohol such as ethylene glycol. In the case of the higher-boiling alcohol, the treated electrode is heated to about 140.degree. C. in an inert gas to drive off the alcohol. The electrodes are then washed in hot hydrogen peroxide solution and then in nitric acid. This NAFION.RTM. impregnation step is followed by hot pressing the electrodes onto an ion exchange membrane for a sufficient time at suitable temperatures and pressures.
Using transfer catalyzation wherein an electrode ink comprising a platinum catalyst on a carbon supporting material is printed on a suitable substrate, such as TEFLON.RTM. or paper, it has been possible to form electrodes containing as little as 0.2 mgm/cm.sup.2 of precious metal. In particular, these electrodes, which are essentially decals formed from a supported platinum catalyst electrode ink are painted or sprayed on the substrate and then dried and hot pressed onto ion exchange membranes. This so-called decal process of applying the electrode ink to the surface of the membrane has been successful but involves the arduous process steps of forming the electrode decal and then transferring it to the membrane.
In all of the foregoing techniques, it has been necessary to utilize liquid-based emulsion and several processing steps to form a film of the electrode material and thereafter bind or press the sheet of electrode material upon the ion exchange membrane, or it has been necessary to use binders and substantial quantities of expensive catalyst materials to prepare membrane and electrode structures. It has also been necessary to utilize large loadings of catalyst to make acceptable electrodes in these prior art methods. The process for preparing the electrodes is inefficient and the reproducibility is poor.
By prior art techniques, it has been impossible to prepare membrane and electrode structures having loadings of the unsupported catalyst materials as low as 3.0 mg per cm.sup.2 or even lower with no compromise in the integrity of the membrane or the performance of the membrane and electrode structure in various fuel cells, gas generating systems and other devices.
U.S. Pat. No. 4,272,353 tries to solve some of these problems by abrading or physically roughening the surface of the membrane to provide a support for locking, uniting or fixing the finely-divided catalyst particles to the surface of the membrane. Particularly, before the catalyst is deposited upon the surface of the membrane, the surface is subjected to a suitable abrading or roughening means. However, the abrasion process can result in deleterious effects to the strength, dimensional stability and electrical properties of the membrane. Moreover, abrading the membrane requires an additional process step.
Moreover, directly applying catalyst inks to a membrane which is in the proton form has been largely unsuccessful. The alcohol carrier causes swelling and distortion of the membrane onto which it is applied.
It is also known to incorporate additives into the ink composition in order to form a suspension of the catalytically active particles and/or binder agents. Additives such as tetrabutyl ammonium hydroxide glycerols and ethylene glycol are known additives which facilitate the printing of the electrode ink onto the surface of the membrane, but such additives adversely interact with many binders and the ion exchange polymers contained in the membrane.
Therefore, a method of making a membrane and electrode structure is needed in which the electrode may be efficiently, inexpensively, and reproducibly applied to an ion exchange membrane, so as to form a uniform electrode structure which uses a relatively small loading of catalyst, does not crack or deform during operation, does not adversely decrease ionic conductivity of the structure, does not effect the strength of the structure and does not adversely interact with the ion exchange polymer contained in the membrane.